1. Field of the Invention
The present invention relates generally to uniformity corrections in lithography systems.
2. Related Art
Conventional lithography systems include, among other things, an illumination system to produce a uniform intensity distribution of a received laser beam. It is desirable that the resulting illumination be as uniform as possible. Illumination uniformity influences the lithography system's ability to pattern uniform line widths across an entire exposure field. Illumination uniformity can significantly impact the quality of devices produced by the lithography system.
In conventional illumination systems, uniformity is never measured during an actual exposure, but can only be measured offline. That is, these conventional illumination systems do not include methods for actively and dynamically controlling illumination uniformity with real time uniformity measurements (or feedback).
Transient uniformity disturbances, have been observed in radiometric illumination tests. Factors, such as beam and component misalignment and/or transmission hysteresis, are example as sources of transient uniformity disturbances. In lithography systems that utilize light wavelengths of about 193 nanometers, for example, transmission hysteresis within the illuminator can cause uniformity to change by up to 0.15% (half of the specification for a typical lithography system) during the space of 90 minutes in some extreme cases. In conventional systems, uniformity of exposure is entrusted to the temporal stability of the optical system.
Conventional techniques for correcting uniformity errors include numerous systems that are well known to those of skill in the art. These illumination uniformity systems are configured to correct uniformity only at a wafer plane. Uniformity corrections are made, re-measured, then saved, and exposures are conducted without further adjustment. Particular correction settings may also be recorded for later use. In short, these systems are only capable of measuring illumination uniformity statically (i.e., off line). These systems do not include capabilities for dynamically measuring illumination uniformity at or near real time.
What is needed, therefore, is a method and system that provides real time uniformity illumination measurements and intensity feedback. More specifically, what is needed is a technique and system to measure illumination uniformity in real time in an image plane.